The present invention relates to a signal receiving apparatus and method and a providing medium.
As a digital broadcast transmitting system using a ground wave, recently, attention is being paid to an OFDM (Orthogonal Frequency Division Multiplex) modulating system. As a service using the OFDM modulating system, in Europe, a radio service using the Eureka 147 DAB (Digital Audio Broadcasting) system has already been started. With respect to a television broadcasting as well, in Europe, the DVB (Digital Video Broadcasting)-T system has already been developed and standardization of the television broadcasting is recommended by the ITU-R (International Telecommunication Union-Recommendation).
In the Eureka 147 DAB system of which service has already been started, since a main service is intended for a mobile unit audio signal, the xcfx80/4 offset differential QPSK (Quadrature Phase Shift Keying) is used for each of carrier waves of the OFDM. Since the system is intended for a mobile unit, resistance to fading is a necessary condition. The system is employed since there is no information in the amplitude direction and it is unnecessary to reproduce an absolute phase.
On the other hand, in the television broadcasting, different from broadcasting service of which target is sound, it is not so much necessary to correspond to be adapted to a mobile unit. Instead, since it is necessary to transmit mainly video information of a large information amount, a high transmission speed is required. Specifically, in the audio service for the mobile unit, very reliable transmission even in hostile environments is demanded. On the contrary, in the television broadcasting service, high speed transmission is demanded. From such a background, in the DVB-T system intended for the television broadcasting service, it is proposed to use a modulating system such as QPSK, 64 QAM, or 16 QAM for modulation of each of carrier waves of the OFDM.
In the ground wave transmission, generally, a multipath exists and the frequency characteristics of a reception signal are distorted by the multipath. It is therefore an important subject to reduce the influence of the multipath. In the OFDM system, therefore, a signal of a copy of a part of a signal to be inherently transmitted is added as a guard interval. By adding the guard interval, with respect to a multipath shorter than the guard interval, a proper signal process is performed on the reception side, thereby enabling the influence of the multipath to be eliminated.
In the OFDM modulating system such as the DVB-T system using modulation of a QAM system as a system of modulating each of carrier waves, when a distortion caused in the multipath occurs, the amplitude and phase of each carrier wave become different from those on the transmission side. It is therefore necessary to equalize (correct) a signal distorted by the multipath so that the amplitude and phase are unchanged. In the OFDM system, the OFDM modulation is carried out by performing the FFT (Fast Fourier Transform) on the reception side. By dispersing pilot signals in transmission signals and by monitoring the amplitude and the phase of the pilot signal on the reception side, the characteristics of a transmission path are estimated and a reception signal is equalized according to the estimated characteristics of the transmission path.
In the DVB-T system, it is proposed that the pilot signals are inserted in a pattern as shown in FIG. 9. In the same figure, the abscissa axis shows a frequency f and the ordinate axis represents a time t. As illustrated in the same figure, in the example, a carrier wave signal for pilot (shown by a black circle) is inserted per 12 carrier waves of one OFDM symbol (its frequency is shown by f0) and the insertion position of the carrier wave signal for pilot is shifted by three carrier waves at every OFDM symbol. A blank circle shows a carrier wave signal for information. Also, a tg denotes a guard interval.
The pilot signals arranged discretely in both the time and frequency directions shown in FIG. 9 were subjected to two-dimensional Fourier transform, the structure of sampling lattice points was examined, and the transmission band width was checked. The result is as shown in FIG. 10. From the same figure, it is understood that the transmission band width when there is no fluctuation in the time direction in the transmission path is within a time corresponding to an interval of three carrier waves. In other words, since there is the transmission band width of ⅓ of effective time of the OFDM symbol (duration of the OFDM symbol except for the guard interval), the pilot signal pattern in the DVB-T system has equalizing capability for the time within ⅓ of the OFDM effective symbol length.
FIG. 11 shows an example of the construction of a conventional signal receiving apparatus for estimating the transmission path characteristics from such a pilot signal and equalizing (correcting) a reception signal. A tuner 2 converts a signal received by an antenna 1 into an intermediate frequency (IF signal) which is outputted to multipliers 3 and 4. Carrier waves whose phases are different from each other by 90 degrees generated by a carrier wave generating circuit 7 are supplied to the multipliers 3 and 4. Each of the multipliers 3 and 4 multiplies the input intermediate frequency signal by the carrier wave, converts the signal into an OFDM signal in a base band, and outputs a resultant signal to an FFT circuit 5. The FFT circuit 5 performs an FFT process on the input signal, thereby OFDM demodulating the OFDM signal in the base band.
An FFT window circuit 6 generates a window as a reference of start of the FFT operation of the FFT circuit 5 by using the correlation of the guard intervals of the OFDM signals from the OFDM signals in the base band outputted from the multipliers 3 and 4 and outputs the window to the FFT circuit 5. The carrier wave generating circuit 7 generates carrier waves whose phases are different from each other by 90 degrees from an output of the FTT window circuit 6 and outputs them to the multipliers 3 and 4.
Each of the carrier waves of the OFDM signal outputted from the FFT circuit 5 is supplied to a dividing circuit 10 and a pilot signal extracting circuit 8 which construct an equalizing circuit 13. The pilot signal extracting circuit 8 extracts the pilot signal from the input signal and outputs it to an interpolating filter 9. The interpolating filter 9 performs an interpolating process on the input pilot signal, thereby estimating the transmission path characteristics of each of the carrier waves of the OFDM signal, and outputs the estimation result to the dividing circuit 10. The dividing circuit 10 divides each of the carrier waves of the OFDM signal input from the FFT circuit 5 by the transmission path characteristics input from the interpolating filter 9, removes the distortion occurred in the transmission path, and outputs a resultant to a demapping circuit 11. The demapping circuit 11 restores the transmission information from a signal point of the signal input from the dividing circuit 10. When an error correcting circuit using a convolutional code or the like exists at the post stage of the demapping circuit 11, a metric to be supplied to a Viterbi decoder is generated by the demapping circuit 11.
A TPS detecting circuit 12 extracts a transfer control signal called a TPS (Transfer Parameter Signal) from an output of the FFT circuit 5. The transfer control signal includes a coding ratio of the convolutional code, a system of modulating the OFDM carrier wave, guard interval information and the like in the next super frame (one super frame consists of eight frames). The TPS detecting circuit 12 controls each of the circuits on the basis of the extracted transfer control signal. For example, the demapping circuit 11 is controlled on the basis of the modulating system of the OFDM carrier wave included in the transfer control signal so as to execute the demapping process corresponding to the modulating system such as QPSK, 16 QAM, or 64 QAM.
The operation will now be described. The tuner 2 converts a signal received by the antenna 1 into an intermediate frequency signal and outputs a resultant signal to the multipliers 3 and 4. Carrier waves whose phases are different from each other by 90 degrees generated by the carrier wave generating circuit 7 are supplied to the multipliers 3 and 4. The transfer wave is generated from the outputs of the multipliers 3 and 4 in correspondence with a phase error detected by using the correlation of the guard interval by the FFT window circuit 6. Each of the multipliers 3 and 4 multiplies the intermediate frequency signal of the OFDM signal input from the tuner 2 by the carrier wave supplied from the carrier wave generating circuit 7 to thereby generate the OFDM signal in the base band, and outputs the resultant signal to the FFT circuit 5. The FFT circuit 5 performs the FFT process on the input OFDM signal in the base band to thereby demodulate the OFDM signal.
The pilot signal extracting circuit 8 extracts the pilot signal from the output of the FFT circuit 5 and outputs it to the interpolating filter 9. The interpolating filter 9 performs the interpolating process on the pilot signal input from the pilot signal extracting circuit 8, thereby detecting the amplitude and the phase component of each carrier wave as transfer path characteristics of the carrier wave and outputs them to the dividing circuit 10. The dividing circuit 10 divides the demodulated signal input from the FFT circuit 5 by the amplitude and the phase supplied from the interpolating filter 9 to thereby eliminate a distorted component caused by the transfer path characteristics. For example, when the amplitude of the carrier wave input from the FFT circuit 5 is xc2xd of the original amplitude, xc2xd is supplied as amplitude information from the interpolating filter 9. When the dividing circuit 10 divides the amplitude of the signal input from the FFT circuit 5 by the amplitude information of the interpolating filter 9, a signal having the original amplitude 1 (=(xc2xd)/(xc2xd)) can be obtained. Similarly, with respect to the phase as well, by performing a complex calculation, a signal having the original phase can be obtained.
The demapping circuit 11 demaps the signal point of the signal outputted from the dividing circuit 10. To this end, the TPS detecting circuit 12 detects the transfer control signal included in the signal outputted from the FFT circuit 5, detects information regarding the modulation system of the OFDM signal from the transfer control signal, and outputs the detection result to the demapping circuit 11. The demapping circuit 11 performs the demapping process in accordance with the modulation system information from the TPS detecting circuit 12 and outputs the processing result.
By the way, in the DVB-T system, as a ratio of the length of the guard interval to the length of the effective symbol length, four kinds of ratios 1/4, 1/8, 1/16, and 1/32 are defined. The guard interval is set (fixed) to the band width of 1/4 of the maximum length so that the interpolating filter 9 can execute the equalizing process even when a signal of the guard interval having any of the four kinds of lengths is received.
As described above, since in the conventional apparatus the band width of the interpolating filter 9 is fixed to 1/4 at which the guard interval is the,longest, when the OFDM signal whose guard interval is shorter than that is received, the signal component band which is originally unnecessary is processed, so that noises increase in association with the signal. There is a problem such that a more accurate transfer path estimating process cannot be realized by the influence of noises.
The present invention has been achieved in consideration of such conditions and is to propose a signal receiving apparatus and a method and a providing medium in which an influence of noises in a transfer path can be more effectively suppressed.
A signal receiving apparatus of the present invention is characterized by comprising a receiving means for receiving a signal transferred in the OFDM system; a demodulating means for demodulating the OFDM signal received by the receiving means; an equalizing means for equalizing the signal demodulated by the demodulating means; a detecting means for detecting the length of the guard interval of the OFDM signal received by the receiving means; and a control means for controlling the equalizing means in accordance with the detection result of the detecting means.
A signal receiving method of the present invention is characterized by comprising a receiving step of receiving a signal transferred in the OFDM system; a demodulating step of demodulating the OFDM signal received in the receiving step; an equalizing step of equalizing the signal demodulated in the demodulating step; a detecting step of detecting the length of the guard interval of the OFDM signal received in the receiving step; and a control step of controlling an equalizing process in the equalizing step in accordance with the detection result in the detecting step.
A providing medium of the present invention is characterized by providing a program which can be read by a computer which allows a signal receiving apparatus for receiving a signal transferred in an OFDM system to execute a process comprising a receiving step of receiving a signal transferred in the OFDM system; a demodulating step of demodulating the OFDM signal received in the receiving step; an equalizing step of equalizing the signal demodulated in the demodulating step; a detecting step of detecting the length of the guard interval of the OFDM signal received in the receiving step; and a control step of controlling an equalizing process in the equalizing step in accordance with the detection result in the detecting step.